2008 Statewide Strategic Plan for Eurasian Watermilfoil in Idaho

and boating activities, with each fragment having the potential to develop into a
new plant. Once introduced, Eurasian watermilfoil can spread exponentially.
E. Growth and Development
Eurasian watermilfoil exhibits an annual pattern of growth. In the spring, shoots
begin to grow rapidly as water temperatures approach 15 degrees centigrade.
Shoots branch profusely when they near the surface, forming a dense canopy.
The leaves below 1 meter senesce in response to self-shading. Plants typically
flower when they reach the surface. After flowering, plant biomass declines as the
result of stem fragmentation. Where flowering occurs early, plant biomass may
increase again later in the growing season, and a second flowering can occur.
During the fall, plants die back to the root crowns, which sprout again in the
spring. In areas with mild winters (such as western Washington and Oregon)
Eurasian watermilfoil frequently overwinters in an evergreen form and may
maintain considerable winter biomass. Eurasian watermilfoil plants do not form
specialized overwintering structures such as turions. Carbohydrate storage occurs
throughout overwintering shoots and roots.
F. Economic Importance
Eurasian watermilfoil is the most important waterweed in the continental United
States, with extremely high annual control costs. States such as Idaho,
Minnesota, Wisconsin, Vermont,
New York, and Washington spend
millions of dollars per year on
Eurasian watermilfoil control.
Figure 9. Eurasian watermilfoil infestation interfering
with recreation
Eurasian watermilfoil adversely
impacts aquatic ecosystems by
forming dense canopies that shade
out native vegetation. Species
diversity and native macrophyte
declines have been reported
associated with invasions by this
aquatic weed. Pure stands of
Eurasian watermilfoil provide poor
habitat for waterfowl, fish, and
other wildlife. Significant rates of
plant sloughing and leaf turnover, as well as the decomposition of high biomass at
the end of the growing season, increase phosphorus and nitrogen in the water
column. Eurasian watermilfoil impacts water control structures, navigation, power
generation and irrigation by clogging infrastructure. Stagnant water created by
Eurasian watermilfoil mats provides breeding grounds for mosquitoes, which carry
West Nile virus. Eurasian watermilfoil interferes with tourism and recreational
activities such as swimming, boating, fishing, snorkeling and water skiing (Figure
9).
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